Teasing!
And if she tries to come into contact with you for what seems to be trivial reasons.

If she treats you differently at all.
If a girl likes you she will always treat you differently from the way she treats other guys. You just need to observe how she interacts with other males and you can figure it out soon enough.

Well, for starters, you're a little too young to thinking this anyway and your priorities should be elsewhere. But, since you have asked this question:

You will know as soon as you make a move. No, I'm not talking about going up and grabbing her. That doesn't work in real life. Talk to her. Make her feel comfortable about being around you. You'll know her interests about you has been peaked if you say something with intention of making her laugh and she bursts out laughing (if she gives that slight giggle with her mouth barely open, you just creeped her out and she no longer has an interest in you). DO NOT MAKE ANY PHYSICAL MOVES. If you just want something quick and go apart, give a slight hint within the conversation. If she wants to, she'll tag along or give a slight hint like biting her bottom lip. If you're not picking up on it, she'll say something along the lines of "you want to go somewhere else?" while giving those "love" me eyes.

If you want a relationship, continue talking and get some basic info about her (no "what's her fav's". Save that for the telephone call). At the end of the conversation, get her number. Wait a day or two to call it. Calling immediately screams you are a love sick creepy stalker to her. If you wait too long, she will be pissed and never want to talk to you again. She'll probably have found someone else by then. Hence, follow the 1-2 day rule. When calling, introduce yourself your name and as the guy that she met at the one place. It will jog her memory and make her feel safe that it's actually you and not some other guy. Talk like you're new friends trying to get to know what each other likes. Be sure to ask how her day is going. ALWAYS LISTEN!!!!!! This is important to establishing the date YOU are creating and whether or not it will go smoothly. Also, don't expect a date to go as you planned. Create a backup plan or just go with the flow. Once the conversation is drawing towards its end, INDIRECTLY ask her for a date (The best line that has time and time again worked: "Hey, you want to hang out sometime?".) If she says yes, she is interested.

Now how to make a date successful is a whole other subject, so I'm going to end it at this. Hope this helps. But, as I said, dating shouldn't be your top priority right now.